The present invention relates to a cold-cathode electron gun serving as an electron source for an apparatus such as a microwave tube as an application of an electron beam and, more particularly, to an electron gun mounted with a field emission type cold cathode with a focusing electrode as a cathode.
The structure of a conventional electron gun mounted with a field emission type cold cathode with a focusing electrode (to be referred to as a cold cathode hereinafter) will be briefly described with reference to FIGS. 4, 5, and 6A to 6C.
As shown in FIG. 4, in a conventional electron gun 31, a conical (trumpet-shaped) Wehnelt electrode 34 with a flange is formed on an electron emission surface 33 of a cold cathode 32, and an emitter electrode 35 with a substantially T-shaped section is formed on the lower surface of the cold cathode 32 on a side opposite to the electron emission surface 33.
The Wehnelt electrode 34 is held as it is fixed with its periphery to a cylindrical support (not shown) arranged around it. The emitter electrode 35 is supported by an emitter electrode support (not shown) through a spring 36. The emitter (not shown) of the cold cathode 32 is connected to an external power supply through the emitter electrode 35 and the emitter electrode support.
The cold cathode 32 is urged by the emitter electrode support and the spring 36 against the central portion of the Wehnelt electrode 34. In other words, the cold cathode 32 is supported as it is sandwiched between the Wehnelt electrode 34 and emitter electrode 35.
The Wehnelt electrode 34 controls the direction of the flow of electrons (electron flow) emitted by the cold cathode 32, and focuses the electron flow. The Wehnelt electrode 34 has an opening 37 formed at its center, and a conical portion 38 formed by bending its portion around the opening 37 conically toward the cold cathode 32. The opening 37 of the Wehnelt electrode 34 passes the electron flow through it, and the distal end of the conical portion 38 is in contact with the cold cathode 32. That portion of the cold cathode 32 which is surrounded by the distal end of the conical portion 38 forms the electron emission surface 33.
The cold cathode 32 has a plurality of emitters 40 formed on the electron emission surface 33 as the surface of the central portion of a substrate 39, and a gate electrode 41 and focusing electrode 42 surrounding the emitters 40, as shown in FIGS. 5 and 6A. The gate electrode 41 is formed on the substrate 39 through a first insulating film 51. The focusing electrode 42 is formed on the gate electrode 41 through a second insulating film 52. Each of the focusing electrode 42, gate electrode 41, and first and second insulating films 51 and 52 is a thin film with a thickness of several xcexcm or less. Gate electrode interconnections 45 for connecting the gate electrode 41 and gate electrode power supply pads 46 on the periphery of the cold cathode to each other are formed under the focusing electrode 42 through the second insulating film 52.
The emitters 40 formed on the cold cathode 32 emit electrons from their sharp distal ends. The gate electrode 41 generates a strong electric field near the emitters 40 to cause the emitters 40 to emit electrons. The gate electrode 41 is connected to an external power supply through the gate electrode interconnections 45 and gate electrode power supply pads 46, and receives power from it. The focusing electrode 42 is connected to another external power supply through the Wehnelt electrode 34, and forms an electric field that focuses the electron flow emitted from the emitters 40.
The gate electrode power supply pads 46 and the external power supply are connected to each other in a space defined between the upper surface of the Wehnelt electrode 34 and the upper surface of the cold cathode 32 by welding bonding wires 43 to the gate electrode power supply pads 46.
The cold cathode 32 operates on the principle of extracting electrons by concentrating a high-voltage electric field (2 to 5xc3x97107 V/cm) to the distal ends of the emitters 40. In order to decrease the operating voltage of the cold cathode 32, the distance between the emitters 40 and gate electrode 41 is preferably as small as possible. The emitters 40 and gate electrode 41 can be designed and manufactured to be close to each other at a distance of as small as on the order of xcexcm by utilizing a thin film process widely employed in the semiconductor field.
The focusing electrode 42 is usually arranged on the gate electrode 41 through the second insulating film 52 with a thickness of about several xcexcm by considering matching with the thin film process described above, although it depends on the design conditions.
In order to apply predetermined voltages to the gate electrode 41 and focusing electrode 42 of the cold cathode 32, terminals to be connected to the corresponding external power supplies must extend from the respective electrodes 41 and 42. Since the focusing electrode 42 is exposed to the surface, the Wehnelt electrode 34 is urged against it from the surface, so that the focusing electrode 42 comes into contact with the corresponding terminal. The underlying gate electrode 41 is connected to the external power supply at a position outside the opening 37 of the Wehnelt electrode 34 in order to maintain the axial symmetry of the electric field in the opening 37 of the Wehnelt electrode 34.
More specifically, the gate electrode interconnections 45 for connecting the gate electrode 41 of the cold cathode 32 to the gate electrode power supply pads 46 serving as the terminals to be connected to the external power supply to each other extend under the focusing electrode 42 from a central emitter area 47 to reach the gate electrode power supply pads 46 formed on the periphery of the cold cathode 32. The gate electrode interconnections 45 and focusing electrode 42 are separated from each other by the second insulating film 52 with a thickness of several xcexcm or less, so that they are insulated from each other.
In the conventional electron gun 31, as shown in FIG. 6B, a contact portion where the Wehnelt electrode 34 is in contact with the focusing electrode 42 extends immediately above the gate electrode interconnections 45. The focusing electrode 42 immediately above the gate electrode interconnections 45 naturally projects from its other portions where the gate electrode interconnections 45 are not present, by a length corresponding to the thickness (t xcexcm) of the gate electrode interconnections 45. Thus, when the conventional Wehnelt electrode 34 with a flat contact surface is brought into contact with the focusing electrode 42, an excessive stress readily acts on the focusing electrode 42 and second insulating film 52 at the projecting portions.
The second insulating film 52 must have a predetermined thickness near the emitters 40 in order to satisfy the focusing characteristics. Accordingly, even if portions of the second insulating film 52 other than near the emitters 40 are to be made thick, it cannot actually have a thickness greatly exceeding several xcexcm. Hence, as shown by a portion P of FIG. 6C, immediately above the gate electrode interconnections 45 and between the focusing electrode 42 and gate electrode interconnections 45, an excessive stress can cause cracking or the like in the second insulating film 52 with a thickness of several xcexcm or less, thus readily destroying it. As a result, the electrical reliability between the focusing electrode 42 and gate electrode interconnections 45 degrades.
It is an object of the present invention to provide a cold-cathode electron gun in which the electrical reliability between the focusing electrode and gate electrode is improved while holding the axial symmetry of the electric field in the opening of the Wehnelt electrode.
In order to achieve the above object, according to the present invention, there is provided a cold-cathode electron gun comprising a cold cathode having an emitter formed on a substrate to emit electrons, a gate electrode formed on the substrate through a first insulating film so as to surround a distal end of the emitter, and a focusing electrode formed on the gate electrode through a second insulating film to correspond to the gate electrode, a conical Wehnelt electrode for connecting the focusing electrode to a first external power supply, the Wehnelt electrode having an opening, at a conical distal end thereof, that comes into with the cold cathode to surround an emitter region including the emitter, the gate electrode, and the focusing electrode, and an undercut formed in a portion of the Wehnelt electrode which is to come into contact with the focusing electrode to correspond to the gate electrode, thereby forming a non-contact portion.